1. Field of the Invention
The present invention relates to a demodulation apparatus for demodulating received input signals, and more particularly, to such a demodulation apparatus which incorporates an adaptive equalizer so that it is suitable for application in a digital mobile communication.
2. Description of the Background Art
In a digital communication involving multipath as in a case of a digital mobile communication, a received input signal is affected by a communication path distortion due to the multipath. In general, in demodulating such a received input signal, a bit error rate can be reduced by detecting after an equalization is performed, using an adaptive equalizer for example.
However, when the equalizer such as an adaptive equalizer is applied to the received input signal transmitted through a path involving only shortly delayed multipath reflections, the bit error rate can actually be increased compared with a case of not using the adaptive equalizer at all.
In fact, under a circumstance in which the communication environment widely changes depending on a location of a receiver, as in a case of mobile communication such as an automobile telephone or a portable telephone, there are locations for which the multipath hardly exist, for instance in a vicinity of a local transmission station, where the use of the adaptive equalizer becomes completely superfluous.
For this reason, there has been a proposition for a selective use of a demodulator incorporating the adaptive filter, as disclosed in Japanese Patent Application Laid Open No. 64-8750.
A demodulation apparatus disclosed in this reference is shown in FIG. 1. In this demodulation apparatus, a distributor 1 distributes a received input signal into two passages, one of which is connected to a first demodulator 4 incorporating an adaptive equalizer, while the other one of which is connected to a second demodulator 6 without an adaptive equalizer. Either one of outputs of the first and second demodulators 4 and 6 is then selected by a signal selection device 7 according to a judgement made by a judging device 9 which judges the output with a smaller bit error rate from the outputs of the first and second demodulators 4 and 6 by comparing a bit error rate value measured by a measuring device 8 with a prescribed threshold value, where the measurement is based on an S/N ratio of the received input signal.
In such a conventional demodulation apparatus, the bit error rate is obtained by comparing a sign of the training signal inserted into the received input signals and the sign of the received input signal.
However, the number of training signals is much smaller than that of the received input signals so that a considerably long period of time is required in order to obtain a sufficiently reliable threshold value, and there is a possibility that a path of large bit error rate may be selected during this period of time.
For instance, for a TDMA using 256 bits per one burst and 15 bit training signal, in order to achieve the bit error rate threshold value of 10.sup.-6 as in the aforementioned reference, at least 10.sup.6 bits are necessary, which means at least 8.times.10.sup.4 bursts are necessary. Even when the threshold value is lowered to 10.sup.-2, several tens of bits are still necessary. Therefore, the time required for determining the threshold value cannot be dismissed as ignorable.
Furthermore, in such a conventional demodulation apparatus, both the first and second demodulators 4 and 6 have to be maintained in activated states throughout the operation of the demodulation apparatus. In other words, the power have to be supplied constantly to both of the first and second demodulators 4 and 6 in such a conventional demodulation apparatus.
This requirement can be a severe limitation for some applications of the demodulation apparatus such as those of automobile telephone and portable telephone in which a battery capacity is rather limited. This problem is practically very important because the power consumption by an equalizer is much larger than other components of the demodulation apparatus.
For instance, in a case of an adaptive equalizer using an RLS algorithm, setting a number of taps to be 5, and taking the coefficients between 3 to 5, because (3 to 5).times.5.sup.2 =75 to 125, at most 123 complex multiplications have to be performed. Since the differential demodulator requires only one complex multiplication, this implies that the adaptive equalizer can consume up to 125 times greater power than the differential demodulator.
Moreover, in this type of a conventional demodulation apparatus, the use of the first demodulator with an adaptive equalizer and the second demodulator without an adaptive equalizer causes a difference in demodulated signal output timing depending on which one of the first and second demodulator is used, such that in the subsequent operations in the demodulation apparatus such as those of the error correction codec and voice codec have to take this difference into account.
Furthermore, a conventional demodulation apparatus incorporating an adaptive equalizer has not been able to produce a good bit error rate performance consistently. The bit error rate performance depends on the equalizer structure and dynamic change of multipath conditions. For example, a conventional adaptive equalizer is not capable of compensating the multipath very well when the multipath condition changes from a non-minimum phase condition to a minimum phase condition. Also, a conventional adaptive equalizer is not capable of adapting to the channel movement when the fading pitch is extremely high. If the equalizer fails to track the dynamic movement of the channel characteristics even once, it cannot continue to compensate the multipath any more, and a correct demodulation cannot be obtained thereafter. The equalizer remains in such a catastrophic state until the next training sequence is received, by means of which the equalizer re-establish the tracking of the dynamic movement of the channel characteristics.
On the other hand, in a more general type of a conventional demodulation apparatus, a frequency offset is generated because of the discrepancy between the transmission frequency of the received input signals and the oscillation frequency of a local oscillator, and this frequency offset have to be removed.
An example of such a conventional demodulation apparatus is shown in FIG. 2.
In this demodulation apparatus, the RF (IF) signals received at an input terminal 11 is multiplied by an output of a local oscillator 12 at a multiplier 13, so as to be converted into baseband signals.
The high frequency components of the obtained baseband signals are then eliminated by a low pass filter 4, while the frequency offset is detected from the output of the multiplier 13 by a frequency offset detection device 16, such that the frequency offset can be removed at a frequency offset removal device 15.
The baseband signals without the frequency offset thus obtained are then equalized by an equalizer 17, and then sent to a demodulation unit (not shown) for performing further processing such as an error correction.
Such a demodulation apparatus is also equipped with a power source unit 18 for supplying activation power to each element of the apparatus.
Now, in this type of a conventional demodulation apparatus, the frequency offset detection device 16 and the demodulation unit have to be maintained in activated states throughout the operation of the apparatus.
As in the previous example, this requirement can be a severe limitation for some applications of the demodulation apparatus such as those of automobile telephone and portable telephone in which a battery capacity is rather limited.
Also, in this case, in order to maintain the frequency offset detection device 16 constantly activated, a huge amount of calculations becomes necessary, which implies that a huge number of gates are necessary in terms of hardware requirement, so that the compact size or large scale integration for the apparatus becomes difficult to realize.
In particular, when the transversal filter type device is used as the frequency offset detection unit 16, the considerations for the power consumption and the amount of calculations can be serious problems practically.